1. Field of the Invention
The present invention relates to a catalyst layer for a fuel cell/battery. This invention also relates to a membrane electrode assembly and a fuel cell, both of which utilize the catalyst layer.
2. Description of the Related Art
In recent years, fuel cells with high energy efficiencies and causing few environmental burdens are attracting attention. Fuel cells electrochemically oxidize fuels such as hydrogen or methanol etc. with an oxidant such oxygen or air and generate electrical energy by transforming the chemical energy of the fuel.
Fuel cells are classified into several types depending on the kind of electrolyte used such as solid polymer type, phosphoric acid type, molten carbonate type, solid oxide type and alkali type. Among them, a solid polymer fuel cell whose electrolyte is a cation-exchange membrane can reduce its internal resistance by using a thinner electrolyte membrane. Therefore, the solid polymer fuel cell can operate a high current to make the fuel cell compact and small. This advantage serves to accelerate the development of solid polymer fuel cells.
The solid polymer fuel cell is generally composed of many stacking unit cells. The unit cell has a sandwich structure in which a membrane electrode assembly is arranged between separators which have flow paths for fuel gases or oxidant gases. The membrane electrode assembly is composed of an anode, a cathode and an electrolyte membrane which is made of a proton conductive polymer and sits between the two electrodes (namely, the anode and the cathode).
One of the key issues for encouraging widespread utilization of solid polymer fuel cells is the reduction of a platinum catalyst which is used in the catalyst layer of the membrane electrode assembly. This is because the world's reserves of platinum are limited. It is said that if all of the present automobiles were shifted from gasoline-powered to fuel-cell-driven, the required platinum would exceed the world's reserves of platinum from the point of view of the required platinum amount per unit area in the present technology. The second reason is cost. It is said that the cost prospect of the membrane electrode assembly is too high to put the fuel cell into practical and widespread use, considering the required platinum amount per unit area in the present technology.
In order to reduce platinum consumption it is essential to use platinum more effectively than ever. The fuel cell reaction takes place at the three-phase boundary in the catalyst layer, where all of the platinum catalyst, the proton conductive material and the fuel gas (or oxidant gas) exist. Thus, it is very important to increase reaction sites by keeping the platinum catalyst and the proton conductive material in an appropriate state in the three-phase boundary. This is achieved, for example, by preparing a catalyst layer by coating and drying ink-like paint which is a mixture of platinum loading carbon and proton conductive polymer electrolyte solution, as is described in JP 2005-320523 A (Laid-Open publication) etc.
FIG. 3 is a partial exemplary diagram showing an example of a conventional catalyst layer. According to the conventional recipe, in which the proton conductive polymer electrolyte (13) and the platinum catalyst loading carbons (14) are combined to form the catalyst layer, the platinum catalyst loading carbons (14) clump together in the electrolyte (13). Then, even if some platinum catalyst loading carbons (11) can provide electrolyte (13) with protons, other platinum catalyst loading carbons (12) can not. Consequently, it becomes difficult to promote efficient use of platinum and this disadvantage results in insufficient battery performance per unit amount of platinum. Particularly when equipped on a vehicle, the fuel gas diffusion and the battery performance tend to be more insufficient because larger instant currents are required than in the case of cogeneration unit use.
The present invention aims to provide a catalyst layer which has a higher output performance but requires less platinum catalyst than before. In addition, this invention provides a fuel cell and a membrane electrode assembly therefor using said catalyst layer.